Optical fiber core wire, method of removing coating from optical fiver core wire and process for producing optical fiber part

ABSTRACT

A resin coated optical fiber  1  comprising a bare optical fiber  2  and sequentially provided on its circumference with a primary coating layer  3   a  and a secondary coating layer  3   b  both made of an ultaviolet-curing urethane resin. The primary coating layer  3   a  and the secondary coating layer  3   b  have respective thicknesses of 60 to 200 μm and 20 to 300 μm. The pulling force for simultaneously removing the primary and secondary coating layers  3   a  and  3   b  is 100 gf or less. The primary coating layer  3   a  has a tensile strength of 0.5 to 1 MPa. The primary coating layer  3   a  after immersed in a solvent has a swelling ratio of 5 to 150%. The secondary coating layer  3   b  has a Young&#39;s modulus of 100 to 1,500 MPa. This resin coated optical fiber allows its coatings to be removed without deteriorating the strength or other properties of the bare optical fiber and without leaving any coating waste on the surface of the bare optical fiber after the coatings were removed.

TECHNICAL FIELD

This invention relates to a resin coated optical fiber, a method ofremoving a coating from a resin coated optical fiber, and a process forproducing an optical fiber part. More particularly, the inventionrelates to a resin coated optical fiber and a method of removing acoating from the resin coated optical fiber, in which the coating of theresin coated optical fiber can be removed without lowering the strengthof a bare optical fiber or other properties and in which, after thecoating of the resin coated optical fiber was removed, there is notrouble that the waste of a coating resin is left on the surface of thebare optical fiber. In addition, the invention relates to a process forproducing an optical fiber part, in which the bare optical fiber clearedof the coating of the resin coated optical fiber is inserted in a thintube such as a ferrule in an optical function part of a fixed attenuatoror the like.

BACKGROUND ART

Generally, the resin coated optical fiber is provided with a bareoptical fiber having a core and a cladding, and a primary coating layerand a secondary coating layer sequentially provided on the outercircumference of the bare optical fiber. In order to inhibit the changein the transmission characteristics of the bare optical fiber due to atemperature change, the primary coating layer is made of a syntheticresin having a Young's modulus of about 1 to 10 MPa at the roomtemperature, and the secondary coating layer is made of a syntheticresin having a Young's modulus of about 400 to 1,000 MPa at the roomtemperature. In the case of the single mode (SM) resin coated opticalfiber, moreover, the primary coating layer and the secondary coatinglayer are formed to have external diameters of about 180 to 200 μm andabout 230 to 250 μm, respectively, on the outer circumference of thebare optical fiber having an external diameter of 125 μm.

Here, the general purpose resin coated optical fiber is designed to fitthe strip force standards (i.e., the strip force of 1.0 N or more and9.0 N or less at 0 to 45° C.) of Telcordia Standards (GR-20-CORE, Issue2), and is made to fit the aforementioned standards, although the resinsticks to the surface of the bare optical fiber (or cladding) after thecoating was removed, if is wiped clean with a paper wiper wetted with asolvent such as ethanol or isopropyl alcohol (IPA).

The coating of the resin coated optical fiber thus constructed has to beremoved in case the bare optical fibers are connected to each other orattached to an optical connector.

As the method of removing the coating of the resin coated optical fiberof this kind, there has been known in the prior art a method of peelingthe coating with a stripper or a method of peeling the coating byimmersing the resin coated optical fiber in a solvent for a long time.

In this method of removing the coating of the resin coated opticalfiber, however, the blade of the stripper may come into contact duringthe coating removal with the surface of the bare optical fiber (or thecladding), thereby to deteriorate the strength of the bare optical fiberextremely. When the waste of the coating resin left on the surface ofthe bare optical fiber (or the cladding) after the coating was removedis to be wiped away with the paper wiper wetted with the solvent such asethanol or isopropyl alcohol (IPA), the bare optical fiber (or thecladding) may have its surface rubbed with the paper wiper. Therefore,the strength of the bare optical fiber may be extremely lowered, asdescribed above. In case the coating of the resin coated optical fiberis to be removed over a length of 300 mm or more from the terminal,moreover, the bare optical fiber is bent by the force applied to theblade of the stripper. Therefore, the bare optical fiber may be brokenbefore the coating is removed. In the method, by which the terminal ofthe resin coated optical fiber is heated and/or immersed in the solventand is pulled by gripping it with a plate-shaped member, moreover, therearises a trouble that the adhesion between the bare optical fiber andthe primary coating layer is so strong that the coating cannot beextracted in a cylindrical shape.

Here will be described the background of a process for producing anoptical fiber part.

Generally, the optical function part such as the fixed attenuator isprovided with the ferrule as the optical fiber part, and the so-called“bare optical fiber” having removed the coating of the aforementionedresin coated optical fiber is inserted into the through hole of theferrule.

FIG. 6 presents explanatory diagrams of a process for assembling anoptical fiber part of the prior art. In FIG. 6(a), a ferrule 10 having athrough hole 10 a of an internal diameter of 126 μm, for example, isused, and the through hole 10 a of the ferrule 10 is filled therein witha (not-shown) adhesive. Next, a resin coated optical fiber 20 has itscoating 20 b removed over a length of 20 to 40 mm from its leading endportion, thereby to expose a bare optical fiber 20 a having an externaldiameter of 125 μm to the outside. Moreover, the exposed outer surfaceof the bare optical fiber 20 a is cleaned by wiping it with alcohol.

Next, the cleaned bare optical fiber 20 a is inserted into the throughhole 10 a of the ferrule 10, and the adhesive is heated and cured to fixthe bare optical fiber 20 a in the through hole 10 a of the ferrule 10.After this, the bare optical fiber 20 a is cut at its protrusions ofabout 3 to 7 mm from the two ends of the ferrule 10.

Next, the ferrule 10 equipped with the bare optical fiber 20 a ispolished at its two end faces, as shown in FIG. 6(c), so that theoptical fiber part can be completed. This optical fiber part isassembled in the housing of the optical function part such as the fixedattenuator so that the optical device is completed.

However, this process for producing the optical fiber part needs thefollowing three steps for producing one optical fiber part: (1) the stepof removing the coating of the resin coated optical fiber to expose thebare optical fiber to the outside; (2) the step of cleaning the bareoptical fiber exposed; and (3) the step of inserting the cleaned bareoptical fiber into the through hole of the ferrule. Thus, this processhas a trouble that the working efficiency is poor. Moreover, the bareoptical fiber having the protrusions of about 3 to 7 mm from the twoends of the ferrule is cut to raise a trouble that the bare opticalfiber is useless.

Thus, there has been devised a method, in which a multiplicity of (e.g.,seven) ferrules 10 are arranged in series with their through holes 10 abeing axially aligned so that the bare optical fiber 20 a is insertedsimultaneously into the through holes 10 a of the ferrules 10, as shownin FIG. 7. In another devised method, as shown in FIG. 8, the bareoptical fiber 20 a is inserted into the through hole 30 a of a longferrule 30, and this ferrule 30 is cut to a predetermined length afterthe bare optical fiber 20 a was inserted.

This method of inserting the bare optical fiber into the ferrule canimprove the working efficiency but has the following drawbacks.

At first, the inserting method shown in FIG. 7 or FIG. 8 has to removethe coating 20 b of the resin coated optical fiber 20 according to thelength of the ferrule 10 or 30 thereby to expose the bare optical fiber20 a to the outside, and to grip the exposed long bare optical fiber 20a with an inserting gripping jig 40. When the long bare optical fiber 20a is to be inserted in this state into the through hole 10 a or 30 a ofthe ferrule 10 or 30, moreover, the bare optical fiber 20 a may bebroken at its portion gripped by the inserting grip jig 40.

Secondly, the waste of the bare optical fiber 20 a sticks to the grippedportion of the bare optical fiber 20 a and migrates into the throughhole 10 a or 30 a of the ferrule 10 or 30 thereby to cause a drawbackthat the characteristics of the bare optical fiber 20 a are degraded.

Thirdly, when the bare optical fiber 20 a is to be inserted into thethrough hole 10 a or 30 a of the ferrule 10 or 30, its outercircumference may contact with the inner circumference wall of thethrough hole 10 a or 30 a of the ferrule 10 or 30 thereby to cause adrawback that the bare optical fiber 20 a is broken. On the other hand,the broken bare optical fiber 20 a is junked to raise a drawback thatthe production yield of the bare optical fiber 20 a drops.

Fourthly, the clearance between the through hole 10 a or 30 a of theferrule 10 or 30 and the bare optical fiber 20 a is hardly left. If thebare optical fiber 20 a is to be inserted in this state into the throughhole 10 a or 30 a of the ferrule 10 or 30, there arises a drawback thatthe insertion of the bare optical fiber 20 a is made difficult by thefrictional resistance between the outer circumference of the bareoptical fiber 20 a and the inner circumference wall of the through hole10 a or 30 a of the ferrule 10 or 30.

Fifthly, the through hole 10 a or 30 a of the ferrule 10 or 30 is mademinute, and the external diameter of the bare optical fiber 20 a is madesmaller than the internal diameter of the through hole 10 a or 30 a.When the bare optical fiber 20 a is to be inserted into the ferrule 10or 30, this insertion has to be performed by observing the through holeof the ferrule 10 or 30 with an enlarging lens. Another drawback is thatthe insertion of the bare optical fiber is difficult unless the throughhole of the ferrule 10 or 30 is tapered.

Therefore, another device for inserting the bare optical fiber into thethrough hole of the ferrule has been devised. In this device, a thinwire or thread is attached with an adhesive to the end portion of thebare optical fiber and is inserted into the through hole of the ferrule,and the wire or thread is extracted from the through hole of theferrule. However, this method has a drawback that the adhesive is peeledoff or sticks thicker than the external diameter of the bare opticalfiber so that the bare optical fiber cannot be pulled out.

The present invention has been conceived to solve the drawbacks thus fardescribed and has a first object to provide a resin coated optical fiberand a method of removing a coating from the resin coated optical fiber,in which the coating of the resin coated optical fiber can be removedover a length of 300 mm or more without damaging the outer surface of abare optical fiber and in which, after the coating of the resin coatedoptical fiber was removed, there is no trouble that the waste of acoating resin is left on the surface of the bare optical fiber. A secondobject of the invention is to provide a process for producing an opticalfiber part, which can prevent the bare optical fiber from being brokenthereby to improve the working efficiency.

DISCLOSURE OF THE INVENTION

In order to achieve these objects, according to a first mode of theinvention, there is provided a resin coated optical fiber comprising: abare optical fiber; and a primary coating layer and a secondary coatinglayer sequentially provided on the outer circumference of the bareoptical fiber. The primary coating layer has a thickness of 60 to 200μm, and a pulling force for simultaneously removing the primary coatinglayer and the secondary coating layer is 100 gf or less.

In the resin coated optical fiber of the first mode, according to asecond mode of the invention, the primary coating layer has a tensilestrength of 0.5 to 1 MPa and a swelling ratio of 5 to 150% afterimmersed in a solvent, and the secondary coating layer has a thicknessof 20 to 300 μm and a Young's modulus of 100 to 1,500 MPa.

In the resin coated optical fiber according to the first or second mode,according to a third mode of the invention, the primary coating layerand the secondary coating layer are made of a synthetic resin.

According to the resin coated optical fiber of the first to third modesof the invention, the coating of the resin coated optical fiber can beremoved without lowering the strength of the bare optical fiber or otherproperties. After the coating of the resin coated optical fiber wasremoved, moreover, there is no trouble that the waste of the coatingresin is left on the surface of the bare optical fiber.

According to a fourth mode of the invention, there is provided a methodof removing a coating from a resin coated optical fiber, comprising:immersing the resin coated optical fiber over a length of at least 300mm from its terminal in a solvent; and simultaneously removing theprimary coating layer and the secondary coating layer after the primarycoating layer was swelled.

According to the method of a fourth mode of the invention of removingthe coating from the resin coated optical fiber, there is no troublethat the waste of the coating resin is left on the surface of the bareoptical fiber after removing the coating from the resin coated opticalfiber, so that the work to clean the bare optical fiber after theremoval of the coating can be omitted unlike the method of the prior artfor removing the coating of the resin coated optical fiber. Moreover,the outer surface of the bare optical fiber is not rubbed to eliminatethe trouble that the strength or other properties of the bare opticalfiber are deteriorated.

According to a firth mode of the invention, there is provided a processfor producing an optical fiber part. When the bare optical fiber is tobe inserted into a thin tube having an internal diameter equivalent tothe external diameter of the bare optical fiber as set forth in any offirst to third modes, the process comprises: connecting a leading fiberhaving a smaller diameter than the internal diameter of the thin tube,to the leading end portion of the bare optical fiber; and inserting andpulling the leading fiber into and out of the thin tube thereby toinsert the bare optical fiber into the thin tube.

In the process for producing an optical fiber part of the fifth mode,according to a sixth mode of the invention, the thin tube is constructedto have a length two times or more of the length to be mounted in anoptical part.

In the process for producing an optical fiber part of the fifth or sixthmode, according to a seventh mode of the invention, the thin tube havingthe bare optical fiber inserted thereinto is cut at a predeterminedlength.

In the process for producing an optical fiber part of the fifth mode,according to an eighth mode of the invention, the thin tube is composedof a plurality of short, thin tubes arranged in series with theirthrough holes being axially aligned.

In the process for producing an optical fiber part of the eighth mode,according to a ninth mode of the invention, the bare optical fiberinserted into the short, thin tubes is cut to the length of the short,thin tubes.

In the process for producing an optical fiber part of any of the fifthto ninth modes, according to a tenth mode of the invention, the leadingfiber is constructed of a quarts glass fiber, and a synthetic resincoating layer formed on the outer circumference of the quartz glassfiber.

In the process for producing an optical fiber part of any of the fifthto ninth modes, according to an eleventh mode of the invention, theleading fiber is constructed of a core, and a cladding and a syntheticresin coating layer sequentially provided on the outer circumference ofthe core.

In the process for producing an optical fiber part of the tenth oreleventh mode, according to a twelfth mode of the invention, thesynthetic resin coating layer is made of an unreleasable syntheticresin.

In the process for producing an optical fiber part of any of tenth totwelfth modes, according to a thirteenth mode of the invention, thesynthetic resin coating layer has a thickness of 5 μm or more.

In the process for producing an optical fiber part of any of the tenthto thirteenth modes, according to a fourteenth mode of the invention,the glass fiber or cladding for the leading fiber has an externaldiameter of 50% or more of the internal diameter of the through hole ofthe thin tube, and the leading fiber including the unreleasablesynthetic resin coating layer has an external diameter of 98% or less ofthe same.

According to the process of the invention for producing the opticalfiber part of the fifth to fourteenth modes of the invention, theexternal diameter of the leading fiber is made smaller than the internaldiameter of the through hole of the thin tube. It is, therefore,possible to insert the leading fiber easily into the through hole of thethin tube and accordingly to insert the bare optical fiber connected tothat leading fiber, easily and without any breakage into the throughhole of the thin tube.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a transverse section showing one embodiment of a resin coatedoptical fiber of the invention.

FIG. 2 is an explanatory diagram showing the state, in which the pullingforce of the resin coated optical fiber of the invention is measured.

FIG. 3 presents explanatory diagrams showing one embodiment of a processfor producing an optical fiber part of the invention.

FIG. 4 is a transverse section of a leading optical fiber in theinvention.

FIG. 5 is a transverse section of another leading optical fiber in theinvention.

FIG. 6 presents explanatory diagrams of a process for producing anoptical fiber part of the prior art.

FIG. 7 is an explanatory diagram of a process for producing anotheroptical fiber part of the prior art.

FIG. 8 is an explanatory diagram of a process for producing anotheroptical fiber part of the prior art.

BEST MODE FOR CARRYING OUT THE INVENTION

Preferred embodiments, to which a resin coated optical fiber, a methodof removing the coating of the resin coated optical fiber and a processfor producing an optical fiber part according to the invention areapplied, will be described with reference to the accompanying drawings.

FIG. 1 is a transverse section of a resin coated optical fiber of theinvention. In FIG. 1, a resin coated optical fiber 1 of the invention isprovided with: a bare optical fiber 2 having a core 2 a and a cladding 2b made mainly of quartz glass; and a primary coating layer 3 a and asecondary coating layer 3 b sequentially formed on the outercircumference of the bare optical fiber 2. Incidentally, the secondarycoating layer 3 b is provided, on its outer circumference, with a(not-shown) coating layer of a thermoplastic resin such as a nylonresin, if necessary.

Here, the primary coating layer 3 a is desired to have a thickness of 60to 200 μm. This reason is described in the following. If the thicknessof the primary coating layer 3 a is less than 60 μm, the adhesion of theprimary coating layer 3 a to the bare optical fiber 2 is more dominantthan the breaking strength of the primary coating layer 3 a. Before theprimary coating layer 3 a is released from the bare optical fiber 2,therefore, the primary coating layer 3 a is broken by the force toremove the primary coating layer 3 a. Still the worse, the waste of thecoating resin is left on the surface of the bare optical fiber 2. If thethickness exceeds 200 μm, on the other hand, the shrinking force of theprimary coating layer 3 a is more dominant than the adhesion of theprimary coating layer 3 a to the bare optical fiber 2 so that theremoving force of the primary coating layer 3 a increases. Moreover, aseparation is caused in the boundary between the bare optical fiber 2and the primary coating layer 3 a by the cooling and the relaxationafter the production, and the strength of the bare optical fiber 2 maydeteriorate.

Moreover, the primary coating layer 3 a is desirably made of such aresin, e.g., an ultraviolet-curing urethane resin as will swell when itis immersed for a predetermined time (e.g., about 30 min.) in an organicsolvent such as a ketone solvent or an alcohol solvent. If the primarycoating layer 3 a is made of such resin, it swells when immersed in thesolvent. Moreover, the solvent migrates into the boundary between thebare optical fiber 2 and the primary coating layer 3 a to weaken theadhesion to the bare optical fiber 2 so that the force to remove theprimary coating layer 3 a can be made weaker than that before theimmersion.

Next, the primary coating layer 3 a after immersed in the solvent isdesired to have a swelling ratio of 5 to 150%. This reason is describedin the following. If the swelling ratio of the primary coating layer 3 aafter immersed in the solvent is less than 5%, the adhesion to the bareoptical fiber 2 does not drop, but the primary coating layer 3 a may bebroken when it is removed. If the swelling ratio exceeds 150%, on theother hand, the tensile strength of the primary coating layer 3 a islowered by the swell, and the primary coating layer 3 a may be brokenwhen it is removed. Here, the swelling ratio (i.e., the swelling ratioof the material in the sheet state) of the primary coating layer 3 a canbe determined from Formula 1.(Swelling Ratio)=((Weight after Swell)−(Weight before Swell))/(Weightbefore Swell)×100[%]  [Formula 1]

Moreover, the tensile strength of the primary coating layer 3 a isdesirably 0.5 to 1 MPa. This reason is described in the following. Ifthe tensile strength of the primary coating layer 3 a is less than 0.5MPa, the primary coating layer 3 a is easily cut by the shearing forcewhile it is being removed. If the tensile strength exceeds 1 MPa, thehardness of the primary coating layer 3 a does not drop even if itswells, so that a high pulling force is required for removing theprimary coating layer 3 a.

Next, the secondary coating layer 3 b is desirably made of a resinhaving a Young's modulus of 100 to 1,500 MPa so that it may be removedin a cylindrical shape. This reason is described in the following. Ifthe Young's modulus of the secondary coating layer 3 b is less than 100MPa, the side pressure on the primary coating layer 3 a is weakened todeform the primary coating layer 3 a thereby to cause a loss or damageeasily on the bare optical fiber. If the Young's modulus exceeds 1,500MPa, on the other hand, the force to swell is inhibited by the secondarycoating layer 3 b so that the force to remove the primary coating layer3 a rises.

Here, the tensile strength and the Young's modulus were measured inconformity with JIS K 7113-1995 by slicing the film sheets, which hadbeen prepared by slicing film sheets cured with an ultraviolet ray of0.35 J/cm2 from the material of the primary coating layer 3 a and thesecondary coating layer 3 b, into test pieces having a thickness of0.20±0.01 mm and conforming to JIS K 7127-1999 (Test Piece Type 5). Thetesting rates of the tensile strength and the Young's modulus at testingwere set to 50 mm/min. and 1 mm/min., respectively, and the strain forthe Young's modulus was set to 2.5%. Moreover, the STROGRPH M2 made byToyo Seiki (Kabushiki Gaisha) was used as the tensile tester. Theterminologies of “tensile breaking strength” and “tensile dividingelastic modulus” are used in the aforementioned JIS standards. In theinvention, for the terminologies of the same meanings, the terminologiesof “tensile breaking strength” in the JIS standards are used as the“tensile strength”, and the terminologies of “tensile dividing elasticmodulus” is used as the “Young's modulus”.

Moreover, the coating thickness of the secondary coating layer 3 b isdesired within a range of 20 to 300 μm. This reason is described in thefollowing. If the thickness of the secondary coating layer 3 b is lessthan 20 μm, the gripping force of the secondary coating layer 3 bpropagates, when removed, to the primary coating layer 3 a. As a result,the primary coating layer 3 a having a soft property is flattened, andan excess frictional force is established at the time of removing thesecondary coating layer 3 b. If the thickness exceeds 300 μm, on theother hand, the primary coating layer 3 a is tightly fastened by thecombination of the Young's modulus and the shrinking force of thesecondary coating layer 3 b, and the adhesion of the primary coatinglayer 3 a rises, whereby the primary coating layer 3 a may be brokenwhen the secondary coating layer is removed.

Next, we prepared the samples by preparing the primary coating layer 3 aand the secondary coating layer 3 b (as will be called together as the“coating 3”) on the outer circumference of a quartz glass fiber having acladding diameter of 125 μm under the conditions enumerated in Table 1,and examined the coating removabilities of the individual samples andthe residence of the waste of the coating resin. In this embodiment, theultraviolet-curing urethane resin is used as both the primary coatinglayer 3 a and the secondary coating layer 3 b. TABLE 1 Swelling Ratio(%) Young's Modulus Thickness (μm) of Thickness (μm) of Sample ofPrimary Coating (MPa) of Secondary Primary Coating Secondary Coating No.Layer Coating Layer Layer Layer 1 10 600 37.5 10 2 10 600 60 10 3 10 60087.5 10 4 10 600 202.5 10 5 10 600 37.5 20 6 10 600 60 20 7 10 600 87.520 8 10 600 202.5 20 9 10 600 37.5 300 10 10 600 60 300 11 10 600 87.5300 12 10 600 202.5 300 13 10 600 37.5 350 14 10 600 60 350 15 10 60087.5 350 16 10 600 202.5 350“Measurement of Pulling Force”

At first, the coating 3 was partially removed by cutting the coating 3circumferentially at a position spaced by 300 mm from the end portion ofeach sample, thereby to expose the bare optical fiber 2 to the outside.A container 4 having an axial length of 500 mm was filled with a solvent5 (25° C. (room temperature)) such as methyl ethyl ketone. Next, eachsample was gripped liquid-tight by means of a coating gripping member 6(e.g., a split silicon rubber member), and the terminal of the resincoated optical fiber 1 was immersed over a length of 300 mm from its endportion 1 a in the solvent 5. After lapse of 30 mins, each sample wasextracted from the container 4, and the outer surface of the coating 3was gripped by a pair of (not-shown) flat members. In this state, thecoating 3 was pulled and removed in the direction of arrow at a pullingspeed of 500 mm/min. The (maximum) force to act was measured as thepulling force. Here, the STROGRPH M2 made by Toyo Seiki Kabushiki Kaishawas used as the tensile tester. A good mark (O) was put when the coating3 was extracted in the cylindrical state, but otherwise a no good mark(X) was put.

In the measurement of the pulling force described above, the immersionlength (i.e., the removal length of the coating) of the terminal of theresin coated optical fiber 1 immersed in the solvent was 300 mm. This isbecause an immersion length less than 300 mm reduces the workingefficiency to insert the bare optical fiber 2 into a long ferrule drops.

“Measurement of Presence or Absence of Waste of Coating Resin”

Next, the outer surface of the bare optical fiber 2 cleared of thecoating 3 was observed with a microscope. The no good mark (X) was putwhen the waste of the coating resin was left on the outer surface of thebare optical fiber 2, but otherwise the good mark (O) was put.

The measurement results are enumerated in Table 2. TABLE 2 Sample Stateof Pulling Presence of Waste of No. Coating Force (gf) Coating Resin 1 X(Core Deformation) X 2 X (Core Deformation) X 3 X (Core Deformation) X 4X (Core Deformation) X 5 X 420 X 6 ◯ 33 ◯ 7 ◯ 27 ◯ 8 X 170 ◯ 9 X 330 X10 ◯ 42 ◯ 11 ◯ 30 ◯ 12 X 150 ◯ 13 X 540 X 14 ◯ 70 X 15 ◯ 55 X 16 X 150 X

From Table 2, it is understood that the pulling force acts as animportant factor to determine whether or not the waste of the coatingresin is left on the outer surface of the bare optical fiber 2 aftercleared of the coating. It is understood from Sample Nos. 6, 7, 10 and11 that the coating 3 can be pulled in the cylindrical state if thepulling force is 100 gf or lower, and that no waste of the coating resinis left on the outer surface of the bare optical fiber 2 after clearedof the coating 3.

Next, samples were prepared by replacing the forming conditions of theprimary coating layer and the secondary coating layer, as numerated inTable 1, by those enumerated in Table 3, and examinations were made onthe coating removabilities of the individual samples and the residenceof the waste of the coating resin. TABLE 3 Swelling Ratio (%) Young'sModulus Thickness (μm) of Thickness (μm) of Sample of Primary Coating(MPa) of Secondary Primary Coating Secondary Coating No. Layer CoatingLayer Layer Layer 17 2 10 90 100 18 2 100 90 100 19 2 1500 90 100 20 21900 90 100 21 5 10 90 100 22 5 100 90 100 23 5 1500 90 100 24 5 1900 90100 25 150 10 90 100 26 150 100 90 100 27 150 1500 90 100 28 150 1900 90100 29 175 10 90 100 30 175 100 90 100 31 175 1500 90 100 32 175 1900 90100

As understood from Table 3, in the case of a swelling ratio of 175%,this excessive swell releases the primary coating layer off the bareoptical fiber 2 before the next step thereby to deteriorate theworkability. In the case of a swelling ratio of 2%, on the other hand,the swell was lacked and the adhesion of the primary coating layer tothe bare optical fiber 2 did not drop so that the coating 3 kept thecylindrical state and could not be pulled out.

Table 4 enumerates the measurement results, which were obtained likebefore by measuring the pulling force and the presence of the coatingresin. TABLE 4 State of Presence of Waste of Sample No. Coating PullingForce (gf) Coating Resin 17 X 840 X 18 X 550 X 19 X 670 X 20 X 700 X 21X 250 X 22 ◯ 70 ◯ 23 ◯ 80 ◯ 24 X 500 X 25 X 360 X 26 ◯ 24 ◯ 27 ◯ 45 ◯ 28X 150 X 29 ◯ 88 X 30 ◯ 60 X 31 ◯ 75 X 32 ◯ 92 X

As understood from Table 4, any of the samples (of Nos., 22, 23, 26 and27) within the range of the invention could be cleared of the coating 3in the cylindrical state, and no waste of the coating resin was left onthe surface of the bare optical fiber 2.

In the aforementioned embodiment, the ultraviolet-curing urethane resinis used as the coating material for the primary coating layer and thesecondary coating layer. However, the invention should not be limited tosuch material within its range.

With reference to FIG. 3 to FIG. 5, here will be described a preferredembodiment, to which a process for producing an optical fiber part ofthe invention is applied. Here, FIG. 3 presents explanatory diagramsshowing a procedure for assembling the optical fiber part of theinvention. In FIG. 3, the portions common to those of FIG. 1 and FIG. 2are omitted in description by designating them by the common referencenumerals.

In FIG. 3(a), numeral 7 designates a long, thin tube made of crystalglass. This thin tube 7 has a through hole 7 a filled throughout itslength with a (not-shown) liquid adhesive. Here, the thin tube 7 has anaxial length of about 100 to 300 mm, and the through hole 7 a of thethin tube 7 has an internal diameter of 126 μm. Moreover, the coating 3(of FIG. 1) constructing the resin coated optical fiber 1 (of FIG. 1) isremoved according to the length of the thin tube 7 so that the bareoptical fiber 2 (of FIG. 1) having an external diameter of 125 μm isexposed to the outside.

Next, the leading end portion of a leading fiber 8, which is maderadially smaller than the internal diameter of the through hole 7 a ofthe thin tube 7, is inserted into one end side of the thin tube 7 andextracted from the other end side of the thin tube 7. The leading fiber8 will be described in detail with reference to FIG. 4 and FIG. 5.

Then, the trailing end portion of the leading fiber 8 and the endportion of the bare optical fiber 2 are fused and connected to eachother, as shown in FIG. 3(c). After this, the leading end portion of theleading fiber 8 is pulled so far that the bare optical fiber 2 ispositioned in the through hole 7 a of the thin tube 7.

In case the leading end portion of the leading fiber 8 is to be pulled,it is desired to grip the resin coated optical fiber 1 (as referred toFIG. 1). This is partly because the bare optical fiber 2 may be broken,if gripped, at its gripped portion and partly because the waste of thebare optical fiber 2, which is attached to the gripped portion, mayenter the through hole 7 a of the thin tube 7 thereby to lower thecharacteristics of the bare optical fiber 2.

Next, the bare optical fiber 2 is inserted into the through hole 7 a ofthe thin tube 7, as shown in FIG. 3(d). After this, the bare opticalfiber 2 is cut at its portions protruding from the two ends of the thintube 7, and the adhesive is heated and set (at 100° C. for about 30 to60 mins.) to fix the bare optical fiber 2 in the through hole 7 a of thethin tube 7.

The thin tube 7 having the bare optical fiber 2 inserted therein is cutto a predetermined length (e.g., to about 16.7 mm in the case of an MUtype fixed attenuator) in accordance with the length of the connector,as shown in FIG. 3(e). Next, thin tubes 71 cut to the length of theconnector are chamfered and polished at their two end portions. As aresult an optical fiber part 72 is completed, as shown in FIG. 3(f).

Here, it is desired that the axial length of the thin tube 7 is twotimes or more of that of the optical fiber part such as the connector.This reason is described in the following. The thin tube 7 such as theferrule is cut to the length of the optical fiber part, in which thethin tube 7 is mounted. If the length of the thin tube 7 is less thantwo times of the length of the optical fiber part, the thin tube 7 canbe applied to only one optical fiber part so that the working efficiencyof the production of the optical fiber part is not improved.

According to the process thus far described for producing the opticalfiber part, the bare optical fiber 2 can be inserted by the single workinto the through hole 7 a of the long, thin tube 7 so that the workingefficiency can be improved. Even unless the inlet hole of the thin tube7 is tapered, moreover, the bare optical fiber 2 can be easily insertedto omit the tapering step.

The foregoing embodiment has described the case, in which the throughhole 7 a is filled in advance with the adhesive. However, this adhesivemay also be applied to the outer circumference of the bare optical fiber2 when this bare optical fiber 2 is inserted into the through hole 7 a.Moreover, the insertion of the bare optical fiber 2 into the throughhole 7 a should not be limited to that into the long thin tube. Forexample, the bare optical fiber may also be inserted into a short, thintube having a length equal to that of an optical fiber part such as anattenuation fiber. Alternatively, a plurality of short, thin tubes arearranged in series with their through holes being axially aligned, asshown in FIG. 7, and the bare optical fiber may be inserted all at onceinto the through holes of those thin tubes through the leading fiber. Inthis case, the plural short, thin tubes are desirably arranged in aV-shaped groove of a bed.

FIG. 4 is a transverse section of a leading fiber to be connected to theend portion of the bare optical fiber 2, and FIG. 5 is a transversesection of a leading fiber in another embodiment.

In FIG. 4, the leading fiber 8 is provided with a glass fiber 81 ofquartz and a coating layer 82 of an unreleasable synthetic resin formedon the outer circumference of the glass fiber 81. The external diameterof the coating layer 82, i.e., the external diameter of the leadingfiber 8 is made smaller than the internal diameter of the through hole 7a of the thin tube 7. Here, the external diameters of the glass fiber 81and the coating layer 82 of the synthetic resin are set at 100 μm and120 μm, respectively.

Next, the leading fiber in another embodiment is provided, as shown inFIG. 5, with a core 81 a made mainly of quartz glass, a cladding 83 amade mainly of quartz glass and formed on the outer circumference of thecore 81 a, and a coating layer 82 a made of an unreleasable syntheticresin and formed on the outer circumference of the cladding 83 a. Here,the core 81 a, the cladding 83 a and the coating layer 82 a of thesynthetic resin are made to have external diameters of 10 μm, 100 μm and120 μm, respectively.

The leading fiber 8 or 8 a thus constructed is provided with the core 81or the cladding 83 a made mainly of quartz glass so that the core 81 orthe cladding 83 a and the bare optical fiber 2 can be connected to eachother. Specifically, the leading fiber 8 or 8 a and the bare opticalfiber 2 to be inserted are set in an external diameter centering typefusing connector so that they can be fused and connected to each otherby melting the two glasses with a discharge. Especially in theembodiment shown in FIG. 5, it is possible to use a core direct visiontype fusing connector having a higher centering precision andaccordingly to reduce the connection fault in the connected portionbetween the leading fiber 8 a and the bare optical fiber 2, that is, theununiformity of the connection areas of the two, the mislocation andmisalignment of the connected portions.

Here will be described the thicknesses of the coating layer 82 or 82 aof the leading fiber 8 or 8 a, the relations between the externaldiameter of the glass fiber 81 (or the cladding 83 a) and the internaldiameter of the thin tube 7, and the reasons for providing the coatinglayer 82 or 82 a on the outer circumference of the glass fiber 81 (orthe cladding 83 a).

At first, it is desired that the coating layer 82 or 82 a made of thesynthetic resin has a thickness of 5 μm or more. This reason isdescribed in the following. If the synthetic resin coating layer 82 or82 a is made to have a thickness less than 5 μm, it is so thin that iteasily becomes so eccentric that the glass fiber 81 (or the cladding 83a) is easily exposed or damaged.

Secondly, it is desired that the external diameter of the glass fiber 81(or the cladding 83 a) is 50% or more of the internal diameter of thethrough hole 7 a of the thin tube 7, and that the external diameter ofthe leading fiber 8 or 8 a containing the synthetic resin coating layer82 or 82 a is 98% or less of the internal diameter of the through hole 7a of the thin tube 7. In other words, it is preferred that the externaldiameter of the leading fiber 8 or 8 a is within a range of 50 to 98% ofthe internal diameter of the through hole 7 a of the thin tube 7. Thisreason is described in the following. If the external diameter is lessthan 50%, the leading fiber 8 or 8 a becomes less rigid and bent in thethrough hole 7 a so that it can hardly be inserted into the through hole7 a. If the external diameter exceeds 98%, the clearance between thethrough hole 7 a and the leading fiber 8 or 8 a becomes small, and thefrictional resistance between the outer circumference of the leadingfiber 8 or 8 a and the inner circumference wall of the through hole 7 abecomes so high as to make it difficult to insert the leading fiber 8 or8 a into the through hole 7 a.

Thirdly, the reason why the synthetic resin coating layer 82 or 82 isprovided on the outer circumference of the glass fiber 81 (or thecladding 83 a) is to protect the outer surface of the glass fiber 81 (orthe cladding 83 a) and to prevent the leading fiber 8 or 8 a from beingbroken when the leading fiber 8 or 8 a is inserted into the thin tube 7.

Fourthly, the synthetic resin coating layer 82 or 82 a is desirably madeof such an unreleasable resin, e.g., an ultraviolet-curing urethaneresin or an ultraviolet-curing epoxy resin as highly adheres of itselfto the glass fiber 81 (or the cladding 83 a) so that it cannot bereleased unless it is mechanically cut or immersed in chemicals such asstrong acid or strong alkali.

From the discussion described above, it is desired that the leadingfiber 8 or 8 a has an external diameter of 73 to 123.5 μm in case theinternal diameter of the through hole 7 a of the thin tube 7 is 126 μm.However, this embodiment is described on the case of the leading fiber 8or 8 a having an external diameter of 120 μm. These dimensional valuesare presented exclusively for the description, and it goes withoutsaying that the invention should not be limited to those values.Moreover, the dimensional values used in the description of theembodiments of FIG. 1 and so on are also presented for the description,and the invention should not be limited to those values.

Here, the foregoing embodiments have been described on the case, inwhich the crystal glass is employed as the thin tube. However, theinvention should not be limited thereto but can employ zirconia, ametal, plastics and quartz. Moreover, the thin tube should not belimited to the arrangement of the MU type optical function part but mayalso be arranged in the FC, ST, SC or LC type connector, for example.Still moreover, the bare optical fiber to be inserted into the thin tubeshould not be the optical attenuation fiber but can also employ a fibergrating to be used in the optical filter, or a quartz bare optical fibersuch as a core-diameter changed fiber or a bare optical fiber having acondensing lens function.

INDUSTRIAL APPLICABILITY

At first, according to the resin coated optical fiber of the invention,the coating of the resin coated optical fiber can be removed withoutlowering the strength of the bare optical fiber or other properties.After the coating of the resin coated optical fiber was removed,moreover, there is no trouble that the waste of the coating resin isleft on the surface of the bare optical fiber.

Secondly, according to the method of the invention of removing thecoating from the resin coated optical fiber, there is no trouble thatthe waste of the coating resin is left on the surface of the bareoptical fiber after the resin coated optical fiber was cleared of thecoating, so that the work to clean the bare optical fiber after theremoval of the coating can be omitted. Moreover, the outer surface ofthe bare optical fiber is not rubbed to eliminate the trouble that thestrength or other properties of the bare optical fiber are deteriorated.

Thirdly, according to the process of the invention for producing theoptical fiber part, the external diameter of the leading fiber is madesmaller than the internal diameter of the through hole of the thin tube.It is, therefore, possible to insert the leading fiber easily into thethrough hole of the thin tube and accordingly to insert the bare opticalfiber connected to that leading fiber, easily and without any breakageinto the through hole of the thin tube. In case the long bare opticalfiber is inserted into either the plural thin tubes of a unit lengtharranged in series or a long thin tube, on the other hand, it ispossible to reduce the working step and to raise the working efficiency.Moreover, the leading fiber is made of the core and the cladding, it ispossible to fuse and connect the leading fiber and the bare opticalfiber by the discharge and accordingly to reduce the connection failure(e.g., the ununiformity in the connection area, or the mislocation ormisalignment of the connected portion) in the connected portion betweenthe leading fiber and the bare optical fiber.

1. A resin coated optical fiber comprising: a bare optical fiber and aprimary coating layer and a secondary coating layer sequentiallyprovided on the outer circumference of said bare optical fiber, whereinsaid primary coating layer has a thickness of 60 to 200 μm and a pullingforce for simultaneously removing said primary coating layer and saidsecondary coating layer is 100 gf or less.
 2. A resin coated opticalfiber as set forth in claim 1, wherein said primary coating layer has atensile strength of 0.5 to 1 MPa and a swelling ratio of 5 to 150% afterimmersed in a solvent and said secondary coating layer has a thicknessof 20 to 300 μm and a Young's modulus of 100 to 1,500 MPa.
 3. A resincoated optical fiber as set forth in claim 1, wherein said primarycoating layer and said secondary coating layer are made of a syntheticresin.
 4. A method of removing a coating from a resin coated opticalfiber, comprising: immersing the resin coated optical fiber as set forthin claim 1 over a length of at least 300 mm from its terminal in asolvent and simultaneously removing said primary coating layer and saidsecondary coating layer after said primary coating layer was swelled. 5.A process for producing an optical fiber part, wherein, when said bareoptical fiber is to be inserted into a thin tube having an internaldiameter equivalent to the external diameter of the bare optical fiberas set forth in claim 1 any, comprising: connecting a leading fiberhaving a smaller diameter than the internal diameter of said thin tube,to the leading end portion of said bare optical fiber; and inserting andpulling said leading fiber into and out of said thin tube thereby toinsert said bare optical fiber into said thin tube.
 6. A process forproducing an optical fiber part as set forth in claim 5, wherein saidthin tube is constructed to have a length two times or more of thelength to be mounted in an optical part.
 7. A process for producing anoptical fiber part as set forth in claim 5, wherein said thin tubehaving said bare optical fiber inserted thereinto is cut at apredetermined length.
 8. A process for producing an optical fiber partas set forth in claim 5, wherein said thin tube is composed of aplurality of short, thin tubes arranged in series with their throughholes being axially aligned.
 9. A process for producing an optical fiberpart as set forth in claim 8, wherein the bare optical fiber insertedinto said short, thin tubes is cut to the length of said short, thintubes.
 10. A process for producing an optical fiber part as set forth inclaim 5, wherein said leading fiber is constructed of a quartz glassfiber, and a synthetic resin coating layer formed on the outercircumference of said quartz glass fiber.
 11. A process for producing anoptical fiber part as set forth in claim 5, wherein said leading fiberis constructed of a core, and a cladding and a synthetic resin coatinglayer sequentially provided on the outer circumference of said core. 12.A process for producing an optical fiber part as set forth in claim 10,wherein said synthetic resin coating layer is made of a synthetic resinhaving a high adhesion to said bare optical fiber.
 13. A process forproducing an optical fiber part as set forth in claim 10, wherein saidsynthetic resin coating layer has a thickness of 5 μm or more.
 14. Aprocess for producing an optical fiber part as set forth in claim 10,wherein the glass fiber or cladding for said leading fiber has anexternal diameter of 50% or more of the internal diameter of the throughhole of said thin tube; and said leading fiber has an external diameterof 98% or less of the internal diameter of the through hole of said thintube.